TECH REPORT: Activation of color centers in glass

MICHAEL OLSEN
Research Glassblower
Colorado State University
Department of Chemistry
Ft. Collins CO 80523-1872
(970) 491-5229 (voice)
(970) 491-1801 (FAX; attn. M Olsen)




Q: WHY DO MANY GLASSES TURN GREEN, BROWN OR BLACK WHEN EXPOSED TO IONIOZING RADIATION?

As a follow-up to my article concerning the role of oxygen in glass, here is an item concerning radiation-induced 'color' in glass. It's a rather technical topic, but I've attempted to bring the chemical and physical discussions down to an intuitive 'kitchen table' level. My intention is to explain how and why radiation causes color and color changes in glass.

The phenomenon of radiation-induced color change is called 'activation of color centers'. The details are quite complex and involve an alteration of the orbital distribution of an atom's valence (outermost) electrons, causing the atom to absorb photons of a different frequency (color) after irradiation than before.

What causes color?

What we describe as the reflected, transmitted, and emitted colors of a material is a consequence of the outermost shell of electrons, the valence electrons of atoms. In the following discussion, we will consider primarily reflected light.

'White' light is actually a mixture of photons of many different frequencies (colors), and color is typically described by wavelength. The valence shell of an atom, described by classical physics as comprised of up to eight discreet electrons, is described by quantum physics to behave as a singular 'thing'. The valence shell will resonate with (it will capture, or absorb) photons of certain discrete energies. This resonance causes the valence shell to become excited, or 'pumped', to a higher energy meta-stable 'level' or state. The atom has a 'ground' (un-excited) state, and may have multiple excited states. Upon absorption of radiation of sufficient energy, electrons may be stripped from the atom, resulting in multiple degrees of ionization, each capable of multiple states of excitation. For example, a hypothetical atom with eight electrons in it's valence shell has its ground state and shall have three meta-stable excited states. Upon capture of a photon of sufficient energy one of the valence electrons is stripped resulting in the atom becoming a singly-ionized ion. This particular ion shall have two meta-stable excited states, and upon capture of a photon of sufficient energy a second of the valence electrons is stripped resulting in a doubly-ionized ion capable of four meta-stable states. And so on. For any monatomic atom, and for the shared valence electron orbitals of any compound, there are these discreet states capable of absorbing (and emitting) discreet quanta of energy which is shuttled around as photons. This is the mechanics underlying what is described below.

Now, let's work with a real example: Let's assume that we have a 'white' light beam composed of red, blue and yellow photons (the primary transmissive colors). We direct this beam onto a puddle of red anthracine dye. Anthracine dyes have tri-cyclic ('aromatic') structures which are capable of resonating with discreet spectra of frequencies. By slight modifications of their molecular structures the valence orbitals of their constituent carbon atoms can be altered, which results in slightly different resonant characteristics. The different anthracine dyes thus appear to our eyes as having different colors. We have selected a red one. The blue and yellow photons in our white light resonate with (are captured and absorbed by) the red dye molecule, and the red photons are scattered (reflected). Some of this scattered red light is directed toward our eyes, and we thus see the puddle of dye as 'red'. If we put the puddle of red dye on a clear film, the red photons will be seen also to pass through the dye.

Now, let's examine a chunk of colored glass. The materials which are added to glass as pigments are selected because their valence shell electrons are capable of selectively absorbing photons of certain frequencies, while passing and reflecting others. Cobaltic oxide for example, will absorb red and yellow but not blue, thus cobalt glass is blue. Transparent glasses in general are a mixture of alkaline and transition metal oxides, most of which are selected precisely because they won't interact with visible light. Put a piece of such a transparent glass in a high neutron-flux or high radiation (such as X-ray) environment, and they will 'become colored' as the glass becomes physically altered at the atomic level. Let's take a look at these processes one at a time.

Neutron capture

An atom consists of a nucleus composed of protons and neutrons, surrounded by electrons organized into multiple layers or shells. The outermost of these is the valence shell, which is responsible for what are loosely called the 'dielectric' properties of the material. The dielectric characteristics involved with color relate to the inter-atomic bonding of atoms, and the absorption of photons. As will be seen, these two characteristics are causitive of some pretty far-reaching and seemingly unrelated effects. If we put a material, in this case a piece of clear glass, into a high neutron flux environment, some of the neutrons will be of a low enough energy (they have been 'attenuated', which is a description of their kinetic energy -- their 'speed') that one or more of them will be captured and meld into the nucleus of an atom. This is how 'isotopes' are created. In some cases what is created is a 'stable' isotope. In other cases, a 'metastable' or 'unstable' isotope is created, in which case the isotope will 'decay', usually by a process called 'beta decay', although 'alpha decay' is also possible.

There is a big difference between what is 'seen' and what has 'happened'. What is seen (in a 'cloud chamber') is that some things (alpha and beta particles) have come whizzing out of the atoms of the target material. What has happened is that some atoms of the target material have undergone a 'transmutation', which is to say that they have actually turned into different elements. What we're talking about here is the nature of 'radioactivity'. The beta particle is actually a liberated electron, and the alpha particle is two protons and two neutrons in a cluster - what is otherwise a helium nucleus (almost all the helium on earth comes from the alpha decay of uranium and thorium deep in the earth). In alpha decay, a 'hunk' of a nucleus 'breaks' away and goes flying off. In beta decay, a single neutron will 'crack' apart into a proton and an electron (and a neutrino). The proton stays with the target atom, but the electron (and the neutrino) go whizzing off. In each of these decays, the target atom has been transmuted into a completely different element, with completely different dielectric characteristics, and may result in a change in color.

X-ray activation

An X-ray is a very short wavelength and very high energy photon, which can so excite an atom's valence electron shell that one or more electrons are completely stripped away. In this new state, the spectrum of incident photons with which the valence shell interacts changes. Presto! The color has changed! In the case of clear glass, the change is from being non-interactive with incident photons to being absorbitive. Thus the glass changes from clear to colored. The irridiated atom is ionized and loses an electron. The liberated electron is now liable to be captured by some other atom within the glass, making yet a 'radical'. It's worth noting that this radical formation process by electron capture can occur with a beta particle (an electron coming from a nuclear decay) in the process described above.

Q: CAN THE ACTIVATION OF COLOR CENTERS BE REVERSED? CAN THE COLORED GLASS BE MADE CLEAR AGAIN?

Yes. Glass, even at room temperature, is a liquid: the interatomic bonds are weak are and constantly breaking and reforming. These bonds are in fact an interaction of (or, sharing of) the valence electrons of adjacent atoms. (In metals this sharing of electrons results in electrical conduction. For example, in a length of copper wire, the individual electrons at one end of the wire will, in theory, eventually migrate to the other end of the wire, due to random motion and without the application of an outside force). The inter-atomic bonding structure within the bulk of a material places physical constraints upon the valence electrons. Therefore, if you activate a color center (by any means) the alteration in the valence shell will be either stable (unchanging with time) or meta-stable (will change gradually with time). The stronger the interatomic structure (or 'lattice' in the case of a true crystal which, unlike a glass, is a solid) the more stable the change. The change can however be reversed by weakening the inter-atomic bonds which will allow the formation of new, lower energy bonds (atoms will break their initial bonds and reform bonds with other neighbors). This can be accomplished simply by the application of heat. In practice, the temperature required for complete color deactivation in an amorphous material (such as a glass) is its annealing temperature. Therefore, simply annealing a piece of glass will deactivate the color centers.

Having worked extensively with researchers who conduct experiments which activate color centers by both of these means, I have frequently had need to deactivate color centers. An unexpected consequence of color center activation is the formation of strained inter-atomic bonds, and unless the piece is annealed, it may spontaneously fracture! As a rule, the more a piece is colored, the more strain the piece is under, and the more it needs to be annealed. Note that the color doesn't 'cause' the strain. Rather, color and strain are two entirely separate issues sharing the same cause.

Finally, it's worth pointing out that color center activation is now so well understood that unusually colored gemstones are now being created by the process. In some cases, semiprecious irridiated stones are fraudulently sold as precious gem stones, and the difference can only be distinguished by a professional gemologist.


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This page 2004 by Michael Olsen. All rights reserved. Permission is granted to link to this page, but you may not copy, modify, publish, or display any of the content of this page, in whole or in part, without the express written permission of Michael Olsen.

Last edited 04-18-03